(1) Field of the Invention
The present invention relates to an engine arrangement, and in particular relates to an arrangement of a four-cycle engine mounted on a snowmobile.
(2) Description of the Prior Art
Conventionally, small snow vehicles such as snowmobiles have mainly used two-cycle engines. A two-cycle engine is characterized by a relatively simple engine configuration having the advantages of being lightweight and compact and still providing high power. However, two-cycle engines have drawbacks, when compared with four-cycle engines, of emitting a greater amount of exhaust and louder noise with greater vibrations.
The recent worldwide trend demands a silent, clean and environment-conscious engine. Snow vehicle engines are no exception and clean and silent engines have been desired. To fulfil this demand, development of engines for snowmobile into a four-cycle configuration has been studied.
As shown in FIG. 1, a typical conventional snowmobile 200 has a body frame 210 in a monocoque frame configuration, wherein an engine 202 is mounted in a space formed between a track housing 211c and a front suspension housing 211b in the front frame (engine mounting frame) 211. For a two-cycle engine, engine 202 is mounted upright in an engine compartment 206.
If a four-cycle engine is to be put into a snowmobile in the same manner as above, the upright mounting has been difficult because a four-cycle engine has a greater full height compared to a two cycle engine due to its bulky cylinder head and the necessity of an oil pan. Further, since a four-cycle engine needs additional unique parts such as an alternator, supercharger, intercooler, oil filter and the like, it has been very difficult to lay out all these parts in a limited engine compartment space.
FIG. 2 shows a proposal to overcome the above space difficulty. In FIG. 2 an engine 202 is inclined forwards in an engine compartment 206 so as to reduce the full height of engine 202 while the aforementioned components are laid out in the space created by the inclined arrangement of the engine.
On the other hand, in order to enhance the power of a four-cycle engine (which is relatively low in power compared to a two-cycle engine), a supercharger (so-called turbocharger) can be provided on the intake side of the engine so as to increase the amounts of air and fuel to be supplied to the combustion chamber, to thereby enhance the engine power.
When the engine has a supercharger, provision of an intercooler in the intake line improves the intake efficiency. However, the position at which the intercooler is attached needs careful thought as to the cooling efficiency of the engine and is affected by the attached position of the supercharger, so that it has been very difficult to design the layout and allot mounting spaces for these components in the engine compartment.
For example, when the intercooler is arranged on the top of the engine, the full height of the engine becomes high so that the center of gravity as well as the body height of the snowmobile become high, hence the result is not preferable.
On the other hand, when the intercooler is arranged in front of the engine, the heavy part is located away from the center of gravity, so that maneuverability decreases. Further, there is another drawback that if the snowmobile collides, the front part of the engine compartment will crush to seriously damage the engine.
For a four-cycle engine, the oil pan, designated at 238, needs to be positioned at the engine bottom. If the engine is of a wet sump type, a large oil pan 238 is needed, which will interfere with the steering shaft, designated at 224, as shown in FIG. 2.
Further, the underside of the engine compartment for a snowmobile serves as a sled sliding over snow, and therefore needs to be closed, which makes maintenance work such as changing oil difficult.
Moreover, when a four-cycle engine with a V-belt type automatic transmission is used for a snowmobile, the engine is used at higher speeds compared to that of a four-wheel vehicle. Therefore, the engine oil increases in temperature due to increased amount of heat from the engine, whereby oil film starvation may occur, leading to damage to the engine.